I'm one of those parents that often invest a lot of time and effort into the Christmas fair; behind the scenes in getting everything organised, setting up, running the actual thing, tidying.
Every year I end up doing a lot more than i was planning to do, simply because I'll see the 'gaps' coming, and rather than let everything fail (which would mean the effort already made would be wasted), I'll jump in and fill the gap, for instance if no-one volunteers for setting up. I have the flexibility to do this and a couple of other parents do the same, so it's alright.
But I am also the parent who will implore other parents to just attend the fair without any feelings of guilt. Because all my (and others') efforts would be wasted if people didn't turn up and spend their money on the things we prepared/manned/tidy up!
Just like there need to be people who bid on the auction items; auctions can make huge amounts of money but if everyone is urged to donate something for the auction and then feels disinclined to bid, it will make less money and create bad feelings rather than making money and creating a good time.
It is like you shouldn't feel guilty for not baking cakes for the cake sales. Please just come and buy some! If you can afford to, of course. I don't mind baking some cakes, but I do mind buying ingredients, baking cakes, and then buying them back off the PTA because not enough people bought cake. If it were like that I'd much rather donate some money and save myself the trouble of baking a nut-free cake; if I want some cake I can always bake one if and when, and can make it the way I like (and with nuts) rather than what will sell best.
All that said, I do get pretty annoyed at how our PTA runs the fair. We have a basically good system with class reps, and each class has one 'job' e.g. Y3 does Santa's Grotto, Y2 does the games room, Y4 does cakes and coffee, ... However they then 'overload' some classes by expecting them to do more than reasonable, leaving the willing helpers overloaded, and eventually frustrated and no longer willing. E.g. if the games room is expected to be such that it takes 4 people 3 hours to set up, then takes 10 people to 'man' it at all times, meaning that EVERY family has to invest an hour, which won't happen, so half the families will be spending two hours or more there (and not spending money elsewhere at the fair), and then takes a good time to tidy up.
Also the PTA is focused on raising money, which is fine but they tend to forget that the fair should ALSO be a fun affair, for the children AND the parents. The idea IMO is to provide a fun event that will also make money. If the fair is such that people only attend because they feel they ought to, you might just as well ask them to donate some money (because they feel they ought to) and save them from having to attend, and save yourselves a huge amount of work!